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Estimating the Number of Turns of an Inductor

Discussion in 'Electronic Components' started by Watson A.Name - \Watt Sun, the Dark Remover\, Jun 5, 2004.

  1. Suppose that I have an inductor that's covered with epoxy or similar
    that prevents me from seeing or finding out how many turns of wire are
    on the core. The core is open, so that it's uncovered and most of the
    magnetic field is outside outside of the inductor. Obviously it's a
    bobbin type core.

    I have measured the inductor with an inductance meter, so I know what
    the inductance and other parameters are.

    Suppose I take some wire, say roughly small if the inductor is small,
    and wind it around the inductor, over the existing windings so that it's
    within the magnetic field. I wind enough wire onto the inductor so that
    I get about 1/9, or 1/16 or 1/25 the inductance in the new coil.

    Since the inductance is the square of the turns, I can say that if I
    have wound 10 turns and the inductance is 1/16th that of the original
    coil, then the turns ratio is 4 to 1, so the original coil is about 40
    turns.

    Obviously the Real WOrld kicks in, and things may not always be exactly
    as they should be. But I haven't tried this, and I'm wondering if any
    other person has, and if it's a not unreasonably accurate[1] way to
    guesstimate the turns, or if it is prone to a large amount of error. I
    guess it would also apply to a toroid if there is enough room to loop
    some wire thru the center hole, but this hole may be filled or covered
    up.

    So has anyone played around with this contrivance?

    [1] A not uncommon journalistic contrivance nowadays; seems like these
    authors just uncan stop not undoing this, and have unremembered to not
    undo it the old fashioned way, and just say "common".

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  2. Tim Williams

    Tim Williams Guest

    I was thinking a solenoid type...obviously you cracked it in half then?
    If you snapped the core back together, the existing winding and your test
    winding would share a good proportion of the flux, as a result it will act
    as a good transformer. However, being open to the air, much of the field
    lines will be lost and you'll have a less than unity coupling coefficient.
    Depending on the frequency, size and turns you may also encounter trouble
    measuring it accurately due to parasitic capacitance in the windings.
    This would be much better because you can get a few turns around the core
    evenly in most cases. Donno about coupling but I imagine it's worse farther
    from the core, even though the turns still circle it fully.
    No, but it's a good idea if you can work around the coupling problems.
    If only I had an L meter...

    Tim
     
  3. John Larkin

    John Larkin Guest

    That is a not-unworthy observation.

    Given a common ferrite bobbin type inductor, you could apply a
    reasonably high frequency sinewave to the inductor, then wind a
    single-turn (or a few, maybe) sense winding over the existing winding,
    then measure the voltage ratio (excitation/sense) to get the turns
    ratio. This only works if the sense winding encompasses as much flux
    as the main winding, which won't be entirely true for a bobbin with
    air return path. It gets better if you can artificially close the gap
    between the ends of the bobbin with some sort of ferrite or
    transformer steel path, sort of a high-permeability c-clamp.

    Hmmm... maybe it's better to apply an external magnetic field to the
    thing to get the ratio. That may make it more likely that the sense
    coil encompasses the same flux as the main coil. Probably so.

    For a torroid of non-silly permeability, this voltage ratio thing just
    works.

    John
     
  4. Rich Grise

    Rich Grise Guest

    Wind turns around it, as you've said. Then drive the unknown core
    with some voltage at a high enough frequency that it can actually
    develop some voltage (so you can measure it); detect and measure
    the voltage at the secondary, (I say detect - depending on what
    freq. you use. I don't know the freq. response of a typical DVM),
    and the ratio is the ratio. :)

    It shouldn't matter if it's a little lossy, because the turns
    ratio is the turns ratio, and the DVM is hi-impedance, right?

    Cheers!
    Rich
     
  5. Phil Allison

    Phil Allison Guest

    "Watson A.Name -in message

    ** One you have got that far you have constructed a transformer. Drive some
    AC current into the original inductor's winding ( from an audio generator or
    similar) and measure the AC voltage on it and on the overwind you created.

    The turns ratio and the (unloaded) voltage ratio you measure are in exact
    proportion.

    The same method can be used to discover the number of turns in the windings
    of a toroidal transformer or any transformer where you can place a small
    overwind.



    ............. Phil
     
  6. Rich Grise

    Rich Grise Guest

    in message

    In fact, I'm willing to bet real money that if you just do the turns
    ratio by volts, that you'll get an integer answer. Or 1/integer,
    don't be a smartass. ;-) I'll bet $100.00 it's within +- 20% of
    the nearest integer (or reciprocal, if you're doing it upside
    down), $10.00 that it's with +-10%, $5.00 for +- 5%,
    and if it's within 1%, we should both win. :)

    Cheers!
    Rich
     
  7. Tom Bruhns

    Tom Bruhns Guest

    So my first obvious question is, why would you care? If you want to
    duplicate the inductor, you already know the inductance, and you can
    measure saturation effects and even loss, with some ingenuity.

    But playing along with your request, if you can wind turns around the
    existing coil, you also have made a transformer. To the extent the
    two windings share a common magnetic field, they will be coupled. You
    can, in fact, measure the leakage inductances and come up with quite a
    good model, and I suppose from that you can deduce the number of turns
    fairly accurately, especially if the coupling is good (and the leakage
    inductance small compared with the coupled inductance) as it would be
    with a ferrite toroid or a pot core or such.

    Cheers,
    Tom

     
  8. Reg Edwards

    Reg Edwards Guest

    If it is an air-cored inductor, calculate the number of turns from its
    measured dimensions.

    This won't work with a ferrite core because its material permeability is not
    known. Although if the core is a simple rod the effective permeability is
    roughly 25 regardless of material permeability.

    With a high permeability core, 100 or more, effective permeability becomes a
    function only of the very long 'air gap'. So inductance stops increasing
    with increasing core material permeabilty.

    But why would you want to know the number of turns if the coil is already
    wound!
     
  9. If you can add turns, put ten turns, and 400Hz 1V for example.
    Measure voltage on original winding.
    If 30V it is 10 x 30 = 300 turns.
    JP
     
  10. Bill Jeffrey

    Bill Jeffrey Guest

    What am I missing here? If you know the inductance of the original
    coil, there are formulas that will tell you the number of turns. Wind a
    coil according to the formula, measure the inductance, and tweak the
    number of turns to get as close as you need to be.

    Bill
    ====================
     
  11. John Larkin

    John Larkin Guest

    As long as the same flux traverses all the turns.

    John
     
  12. pepepepepepepedantic
     
  13. After reading several followups so far, I'm getting the picture that it
    would be easier to measure the voltage ratio. Rich suggested using a
    DVM, but IIRC their AC bandwidth is limited, and drops off above a few
    kHz or so. Rectifying the AC is an alternativce, but then it's not
    accurate if the .6V diode drop is a considerable part of the rectified
    DCV. An O'Scope seems the best way to measure, if it can be calibrated.
    Actually, come to think of it, all that's needed is the ratio, not the
    absolute V values.

    One thing that I had in mind when I originated this idea was that, say
    for instance, I'm measuring a trigger transformer for a xenon tube,
    where the number of turns could be thousands. If I wound a few tens of
    turns on it, the V ratio could be a hundred or more. That might be a
    bit more difficult to measure than the inductance.

    Thanks to all for the thoughtful responses. I'm going to have to try a
    few experiments to see how these work.
     
  14. Thanks for the interesting info. I would expect the core to be more of
    a bobbin. But when it's covered, it's not always certain.
    If I don't know the number of turns to begin with, do you expect me to
    UNwind the coil to find the number of turns?

    As I said, the coil is usually covered or potted in epoxy.
     
  15. Okay, I have two identical adjustable core coils, one with the slug all
    the way in and the other all the way out. The Out one measures 100 uH
    and the In one measures 180 uh. I put both into a box, each with
    terminals to the outside, so that the physical coil can't be seen. Then
    I give them to you along with the inductance of each, and you tell me
    that, by your formulas, the Out one has a different number of turns than
    the In one????

     
  16. One way might be to in-case the coil in epoxy resin and saw it in half and simply count the windings.
     
  17. normanstrong

    normanstrong Guest

    The voltage ratio of a transformer is the same as the turns ratio.
    Therefore, wind ten turns around the inductor and measure the voltage
    at the output when a known voltage is applied to the inductor.

    Norm Strong
     
  18. Reg Edwards

    Reg Edwards Guest

    The voltage ratio of a transformer is the same as the turns ratio.
    ====================

    Agreed. That's about the best he can manage. But what is not known is the
    coefficient of coupling between the two coils. They are not wound in the
    same volume of space or anywhere near to it. One is entirely outside the
    other.

    If the outside coil has a coefficient of coupling of 0.5 with the inside
    coil then it is equivalent to a coil with only half the number of turns.

    The arithmetic is simple. But what the coeff of coupling might be is
    anybody's guess without knowledge of ALL dimensions of BOTH coils. Ask your
    dentist if you could borrow his X-ray machine for the day. Even then a
    hefty treatise involving higher mathematics on how to calculate the
    coefficient of coupling between two coils would be essential.

    All one knows is that the turns error, possibly very large, must lie on the
    low side of the true value.

    Its just occurred to me that with access to a precision X-ray machine or
    electron microscope it may be possible actually to count the number of
    turns. Try NASA.

    How many Henrys is the thing anyway?
    ===
    Reg.
     
  19. A lot of our larger battery charger transformer designs have primaries wound
    outside with the secondaries closest to the core. It is quite a common
    technique, even on some of the smaller stuff we use.

    Peter
     
  20. Reg Edwards

    Reg Edwards Guest

    A lot of our larger battery charger transformer designs have primaries
    wound
    =================================

    Of course they are. They do it all the time. Ever since the Victorian Age.
    But the primary-to-secondary coefficient of coupling, with the leakeage
    reactances, is accurately KNOWN from the start of the design. Certainly not
    the last. It's fundamental. But being at least acquainted with the things,
    I would have thought you already knew that.

    But students should not take me too seriously. I'm really a kindly person.
    ===
    Reg
     
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